Coatings may be applied to medical devices to provide certain advantages or functionality. Medical devices may be coated so that the surfaces of such devices have desired properties or effects. For example, medical device coatings may provide localized delivery of therapeutic agents to target locations within the body, such as to treat localized disease (e.g., heart disease) or occluded body lumens. Localized drug delivery may mitigate undesirable side effects or costs of systemic drug administration. Additionally, localized delivery of a therapeutic agent from a coating may provide a higher concentration of therapeutic agent at a specific point of treatment than would otherwise be achievable by systemic administration. Localized drug delivery may be achieved, for example, by coating endovascular devices such as balloon catheters, stents and the like with the therapeutic agent to be locally delivered. The coating on medical devices may provide for controlled release, which may include long-term or sustained release, of a bioactive material.
For certain medical applications, a coating containing a therapeutic agent is applied to the external surface of an endovascular medical device. The medical device may be configured to bring the coating into therapeutically effective contact with the wall of a body vessel. For instance, the medical device may be a radially expandable tubular stent formed by a plurality of interconnected members defining open cells extending between an external (abluminal) surface and an internal (luminal) surface. A releasable therapeutic agent may be applied to the abluminal surface of the stent for delivery to a treatment site within a body vessel. The luminal surface defines a tubular lumen extending axially from the proximal end to the distal end of the stent. Such coated stent structures are commonly deployed within a body vessel to maintain patency of a stenosis, and the therapeutic agent may be selected to mitigate or prevent restenosis of the body vessel after dilation. For example, the stent may be delivered endovascularly using a catheter delivery system by expanding the stent from a radially compressed delivery configuration within a portion of the catheter to a radially expanded configuration within the body vessel. The stent delivery may be performed as part of a procedure to dilate a blood vessel with the catheter balloon, such as percutaneous transcoronary angioplasty (PCTA). The stent may be radially expanded by a balloon attached to the catheter or may be formed of a material that radially self-expands when released from the catheter.
For many such medical procedures, coated endolumenal devices are preferably coated on the abluminal surface with a particular therapeutic agent in a manner that provides a uniform coating and minimizes coating of the luminal surface. In addition, the therapeutic agent is preferably localized on the interconnected members (e.g., struts and bends) of the stent, rather than being present within the open cells between these members. Upon radial expansion of the endolumenal device, the distance between adjacent members typically increases and the area enclosed by the open cells between these members typically increases. As such, therapeutic agent coated over, or bridging, such open cells may fall through the cells, into the lumen and be undesirably washed away from the point of treatment without contacting the wall of the body vessel. Therefore, coating methods that localize application of the therapeutic agent to the desired coating surfaces of the endolumenal medical device are particularly desirable.
Coatings have been applied to medical devices by processes such as dipping, spraying, vapor deposition, plasma polymerization, and electrodeposition. Although these processes have been used to produce satisfactory coatings, they have numerous, associated, potential drawbacks. For example, it may be difficult to achieve coatings of uniform thicknesses, both on individual parts and on batches of parts. Also, these coating processes may require that the coated part be held during coating, which may result in defects such as bare spots where the part was held and which may thus require subsequent coating steps. Further, many conventional processes require multiple coating steps or stages for the application of a second coating material, or to allow for drying between coating steps or after the final coating step.
One method of coating an endoluminal medical device involves mounting the endoluminal medical device on a mandrel and spraying a solution of a therapeutic agent in a volatile solvent onto the abluminal surface of the mounted endoluminal medical device. The solvent is allowed to evaporate, leaving the abluminal surface coated with the therapeutic agent. Optionally, a polymer may be dissolved in the solution with the therapeutic agent and solvent, or applied with the solvent to form a separate coating layer from the therapeutic agent. When the endoluminal medical device is a tubular radially expandable structure, such as a stent, the medical device is typically mounted on the mandrel in a radially expanded position including a plurality of openings. One difficulty with the above-described method of coating the stent is the potential for coating defects. While some coating defects can be minimized by adjusting the coating parameters, other defects occur due to the nature of the interface between the stent and the mandrel on which the stent is supported during the coating process. A high degree of surface contact between the stent and the supporting apparatus can provide regions in which the liquid composition can flow, wick, and collect as the composition is applied. As the solvent evaporates, the excess composition hardens to form excess coating at and around the contact points between the stent and the supporting apparatus, also referred to as “webbing” of the coating. Upon the removal of the coated stent from the supporting apparatus, the excess webbed coating may stick to the apparatus, thereby removing some of the needed coating from the stent and leaving bare areas. Alternatively, the excess coating may stick to the stent, thereby leaving excess coating as clumps or pools on the struts or webbing between the struts. During implantation of the coated stent, excess therapeutic agent deposited within the openings in the stent frame may be dislodged upon radial expansion of the coated stent and fall through the openings into the lumen of the stent.
Thus, there is a need for coating methods and structures useful to minimize the interface between the stent and the apparatus supporting the stent during the coating process to minimize adverse coating defects. Accordingly, the present invention provides for a device for supporting a stent during the coating application process. The invention also provides for a method of coating the stent supported by the device.